The present invention relates to an apparatus and method for measuring the time evolution of carriers (electrons and holes) propagating within submicron and micron electronic devices.
Electronic devices used in computers and communication systems are continually becoming both smaller in physical size and faster in execution capabilities. With technology progressing at its present rate, it is not inconceivable that submicron and micron sized electronic devices will soon become commonplace in the computer and communications industries. While submicron and micron sized electronic devices bring innumerable benefits to these industries, their extremely small size creates problems with regard to testing. For one thing, the size of submicron electronic device makes it virtually impossible using conventional state-of-the-art technology to observe directly the ultrafast (frequently in the picosecond and femtosecond regimes) kinetic activity and pathways of carriers present within such a device. This is because the current technology uses light to probe the spatial distribution of the carriers and light is limited in resolution to a few microns. Nonetheless, despite present day technological inadequacies, there persists a definite need to know the speed and pathways of carriers propogating in submicron and micron electronic devices.
In the past, electron microscope have frequently been used to explore spatial relationships in the submicron and micron world.
Electron microscopes are traditionally classified as either transmission or scanning instruments.
In a transmission electron microscope an electron beam, which is generated by an electron gun, is focused by a magnetically operated condenser lens onto the specimen. The specimen is usually mounted in a mechanical stage, which forms a part of a magnetically operated objective lens. The objective lens forms an intermediate image of some 50 to 100 times magnification of the specimen in the vicinity of a magnetically operated projector lens, which further magnifies the image and projects the magnified image onto an observation screen for viewing.
In a scanning electron microscope, which is very similar in operation and construction to a closed circuit television system, a finely focused electron beam is formed by an electron lens system. The beam sweeps rapidly across the specimen, stimulating the emission of secondary electrons from the area it strikes. The secondary electrons are then collected to produce a signal which is then amplified. The amplified signal is then used to vary the intensity of a second electron beam as it scans a cathode ray tube in synchronism with the first beam to produce a light image of the specimen.
One drawback to the use of electron microscopes is that, when used in the manner described above, they lack the independent capability to temporally resolve ultrafast events. This makes electron microscopes, when used alone, unsuitable for measuring and recording the ultrafast time evolution of carriers within submicron and micron electronic devices.
Streak cameras, are a well known type of instrument which have been used to directly measure the time dynamics of luminous events. A typical streak camera includes an entrance slit which is usually rectangular, a streak camera tube, input relay optics for imaging the entrance slit onto the streak camera tube, sweep generating electronics, and output relay optics for imaging the streak camera image formed at the output end of the streak camera tube onto an external focal plane. The image at the external focal plane is then photographed by either a conventional still camera or a television camera. The streak camera tube generally includes a photocathode, an accelerating mesh, sweeping electrodes, and a phosphor screen. The streak camera tube may also include a microchannel plate. In the operation of a streak camera, light incident on the photocathode is converted into a streak image, which is formed on the phosphor screen with the intensity of the streak image from the start of the streak to the end of the streak corresponding to the intensity of the light incident thereon during the time window of the streak. The time during which the electrons are swept to form the streak image is controlled by the sweep electronics which supplies a very fast sweep signal to the sweeping electrodes. The input optics of the streak camera may comprise a single lens or a plurality of lenses.